Kasra Ghahremani

Quality assurance for high-frequency mechanical impact (HFMI) treatment of welds using handheld 3D laser scanning technology

The idea of using 3D point clouds obtained with the aid of a handheld 3D laser scanner for the quality assurance of high-frequency mechanical impact (HFMI) treatment is proposed and demonstrated in this paper. The effectiveness of impact treatments for extending the fatigue lives of welded structures has been demonstrated in numerous studies. Guidelines for the proper execution of impact treatments have been developed. A lack of suitable quality assurance (QA) procedures for accepting or rejecting the treatment after completion has been previously identified. In contrast with the existing QA procedures, which are based mainly on controlling inputs and visual inspection, a technology-based, quantitative methodology is developed in this paper. Five welded specimens were subjected to impact treatment at various levels to simulate under-, proper, and over-treatment. A handheld 3D laser scanner was then used to facilitate a point cloud-based method to determine the geometric parameters of the treated weld toe groove, which were then measured manually. The results show that the proposed methodology is successful in identifying the different treatment levels. This approach has a number of advantages over the existing QA methods, including the following: providing quantitative measures, ease of use, and archive-ability.

Fatigue Testing and Structural Health Monitoring of Retrofitted Web Stiffeners on Steel Highway Bridges

Numerous steel highway bridges, still in use today, were built during the construction boom between the late 1950s and the late 1970s. Fatigue cracking can be considered a main source of deterioration for these bridges. The largest category of observed fatigue cracks is caused by out-of-plane distortion. The most susceptible locations are those at which transverse structural components (such as diaphragms or cross frames) are framed into longitudinal girders through web stiffeners that are not attached to the flanges. In the current study, a web stiffener detail is fatigue tested under different cyclic loading conditions. As-welded specimens are tested, along with specimens retrofitted by grinding and rewelding, needle peening, or the adhesive bonding of fiber-reinforced polymer attachments. Direct strain and deflection measurements are compared with finite element analysis predictions, and local (hot-spot) stresses are compared with hot-spot stress design curves. A time series–based method for damage detection is also explored for the prediction of fatigue crack depth with strain data. The method is validated through the use of small- and large-scale specimen strain data. It is found that damage measures based on strains in the vicinity of the critical hot spot are closely correlated with the true crack depth.

Inhibiting Distortion-Induced Fatigue Damage in Steel Girders by Using FRP Angles

AbstractThe idea of retrofitting web stiffener ends in steel bridge girders susceptible to distortion-induced fatigue using adhesively bonded fiber-reinforced polymer (FRP) angles is introduced in this study. The proposed retrofit method is relatively cheap and easy to use and does not require deck removal or any other severe modification to the steel girder. Fatigue tests were conducted on specimens designed to model the conditions in the region between a web stiffener and a flange in a steel girder bridge. Fatigue life increases on the order of several hundred percent were achieved by implementing the proposed retrofit. The effect of the retrofit on the hot-spot stress in the critical weld detail can be seen in the experimental strain data and in a subsequent finite-element (FE) analysis of the tested specimen geometry. Further research is recommended to assess the performance of the retrofit on full-scale girders and to develop guidelines for fatigue verification and design of the FRP angle and adhesiv...

Rail corrosion forensics using 3D imaging and finite element analysis

Rail infrastructure renewal maintenance is capital intensive. As a contributor to rail deterioration, corrosion damage needs to be accurately analysed for renewal maintenance planning. The main contribution of this study is to introduce an information-dense forensic analysis method for characterizing rail corrosion damage in situ based on 3D imaging. Two state-of-the-art technologies, an arm laser scanner and handheld laser scanner, are employed for onsite digitization of the rail surface. Acquired 3D image data is analysed to characterize pitting corrosion in terms of volume, surface area coverage and average pit depth. Cyclic loading of the sampled rail is simulated using finite element analysis of the 3D image to establish risk potential for crack initiation. A case project was used to validate the feasibility of the developed approach. The results of this study demonstrate the usefulness of applying forensic methodology to renewal maintenance planning.

Fatigue Testing and Finite Element Analysis of Bridge Welds Retrofitted by Peening under Load

Residual stress-based post-weld treatments such as needle peening, hammer peening, and ultrasonic impact treatment (UIT) offer a promising means for extending the fatigue lives of existing welded highway bridges. When applied to bridge welds in service, these treatments can be particularly effective, since the stresses due to the self weight of the bridge have already been imposed. In this paper, a finite element (FE) analysis study is performed to investigate the additional benefit that may result from applying post weld-treatments under load. Fatigue tests of small-scale weld specimens treated with and without preloading are first described. 2D FE models that simulate the treatment process are then described and used to model the treatment of the fatigue specimens. Effects of plate thickness, indentation depth, and preload level on the residual stress distribution induced by peening under load are then studied. Based on the results of this work, recommendations are made to aid in the prediction of the fatigue performance of bridge welds retrofitted by peening under load.

Erratum to: Testing and fracture mechanics analysis of strength effects on the fatigue behavior of HFMI-treated welds

In this paper, recent fatigue tests conducted on welded specimens subjected to high frequency mechanical impact (HFMI) treatment are described, geometry measurements and metallurgical analyses of the tested specimens are presented, and efforts to estimate the test results using a nonlinear fracture mechanics model are discussed. The specimens were fabricated from 9.5-mm-thick (3/8 in.) aluminum (5083-H321) and high-strength steel (ASTM A514) plate. The specimen geometry and preparation followed procedures used in previous studies on mild steel (CSA 350W). Fatigue tests were performed on the as-welded and impact-treated specimens under two loading histories (constant amplitude with and without periodic under-loads) at several equivalent stress ranges. Residual stress distributions were determined by x-ray diffraction. In addition, weld toe geometry measurements were obtained using silicon impressions and micro-hardness distributions were obtained on polished weld samples for each material type. This information was used to establish parameter values for a nonlinear fracture mechanics analysis. The employed fracture mechanics model is reviewed in this paper, and its benefits as a tool for modelling the fatigue behavior of impact-treated welds are discussed. Following this, the effectiveness of the model in estimating the test results for the three materials is assessed.

Fatigue Retrofitting of Web Stiffeners in Steel Bridges Using Pultruded FRP Sections

A new retrofit method for the distortion-induced fatigue problem in steel bridges by using adhesively-bonded fiber reinforced polymer (FRP) angles is developed in this paper. This retrofit method is relatively cheap and easy to use and does not require deck removal or any other modification to the steel girder. The FRP retrofitted specimens are found to have significantly longer fatigue lives than as-welded specimens and specimens repaired by means of two other conventional repair methods. The hot-spot stress method is used to quantify the effectiveness of the proposed retrofit method. A coarse finite element (FE) analysis is used to predict the effectiveness of the proposed retrofit methods in terms of the reduction in the hot spot stress value, and analytically calculated hot-spot stresses are validated with directly measured values. The effects of a number of the varied geometrical and mechanical parameters on the efficiency of the proposed retrofit are then studied using an FE analysis and recommendations are made to improve the efficiency of this FRP-based retrofit method.